Certain embodiments of the present invention relate to a diagnostic X-ray system which measures and images anatomical structures. More particularly, certain embodiments relate to methods and apparatus to reduce signal conversion time for a solid-state panel of the X-ray system in order to increase frame rate.
Within the field of diagnostic X-ray imaging, large area solid-state X-ray detectors have been developed in the X-ray art. Such a detector typically comprises a scintillating layer in contact with an array or panel of photodiodes, each with an associated field-effect transistor (FET) acting as an electronic switch. The photodiodes are initially charged by connecting them to a known stable voltage by activating the FETs. Subsequently, the photodiodes are isolated by turning off the FETs. Upon exposure to X-rays, the scintillator produces light which discharges each photodiode in proportion to the X-ray exposure at the position of the diode. The diodes are then recharged by again connecting them to the known stable voltage. The charge used to restore the diode to its initial voltage is measured by a sensing circuit, and the value is digitized and stored.
In such a detector, the photodiodes and their associated FETs are typically organized in rows and columns. The gates of the FETs along a row are connected together, and the row electrodes are connected to scanning electronics. During read-out of the detector, rows of FETs are turned on sequentially, and an entire row of detector elements is read out at the same time. Because of imperfections in the FETs, a time-dependent background current is generated when the FETs are turned on and off. The result is an offset signal that is unrelated to X-ray exposure. The offset signal is typically referred to as switching charge retention. Since the rows are read sequentially, a portion of the switching charge retention is row correlated, i.e. the switching charge retention is roughly the same for all elements in a given row, but varies from row to row. But to make matters more complicated, the switching charge retention for a given row changes with the frame rate of the imaging system.
Also, there are other offset signals that are generated due to both the photosensitivity of the FETs and the capacitance between the photodiodes and the data lines. When light hits the array, the FETs tend to conduct and, much like in normal operation, exhibit charge retention at the conclusion of the X-Ray exposure. Furthermore, as the photodiodes discharge, the capacitance between the photodiode and the data line also experiences a change in charge. Due to the resistance of the data line, the effect may take some time to settle, looking like another source of signal while the effect decays. The composite offset signal, due to exposure, may be referred to as photoconductive charge retention.
The switching charge retention and composite photoconductive charge retention combine to create an offset signal in the elements of the array that should be accounted for during read-out. In the absence of the offsets, the converting circuitry would require only the dynamic range and resolution of signals generated by normal X-ray exposure. In practice, however, the dynamic range of the offset signals may be larger than the dynamic range of useful signals for imaging. For practical reasons, converting circuits have limitations in input signal dynamic range, conversion resolution, and conversion speed. In the absence of compensation for the offsets, the converting circuitry would be required to accommodate an increased input dynamic range without sacrificing resolution and speed.
Previous efforts to solve the problem of charge retention have only accounted for switching charge retention and relied on a prior calibration of the switching charge retention for a constant frame rate as in U.S. Pat. No. 5,604,347 to Petrick et al. Initially, a calibration is performed to measure the average offset of each row. Subsequently, an offset compensation value for each row is stored in the memory of a converting circuit. The stored compensation values are added to the incoming signals during operation of the detector. However, the method does not properly compensate for the contribution of photoconductive charge retention nor an imaging system where the frame rate is varying.
A need exists for an approach to reduce signal conversion times by adjusting for row-to-row variations caused by both switching charge retention as a function of frame rate and photoconductive charge retention as a function of X-Ray photon flux in order Summary of Invention to increase imaging frame rate.
Summary of Invention
Embodiments of the present invention provide an X-ray system for generating and displaying a plurality of image frames corresponding to internal structure within a subject such that signal conversion time is reduced, thus increasing frame rate. The diagnostic X-ray system comprises an X-ray tube for generating X-ray signals, a solid-state detector module responsive to the X-ray signals and an image processing module generating a plurality of normalized detector signals for a current image frame. The normalized detector offset signals for the current frame are dynamically adjusted for row-to-row variations in charge retention of a detector panel array of the solid-state detector module as frame rate changes, and as the X-Ray photon flux changes from frame to frame.
Apparatus is provided to reduce signal conversion time of an X-ray system in order to increase frame rate. The X-ray system includes a scintillator converting X-ray photons to light photons, an array of photodiode/field-effect-transistor pairs abutting the scintillator and being responsive to the light photons to affect discharge of the array, and read-out electronics to read a current row of the array. The read-out electronics are connected to columns of the array and are responsive to charge. The readout electronics are used to generate a set of normalized detector signals such that the set of normalized detector signals is adjusted for variations in signal strength caused by temporal row-to-row and frame-to-frame variations in charge retention in the array.
A method is also provided to minimize signal conversion time for a solid-state detector panel of an X-ray system in order to increase frame rate. A measurement of a set of induced signal offsets caused by time varying charge retention associated with the detector panel is performed during a phantom time segment prior to normal signal readout of the detector panel for a current image frame. A set of adjustment values is generated in response to the set of induced signal offsets. Subsets of signal values of the detector panel are detected and normalized to a pre-determined signal dynamic range as part of normal signal readout of the detector panel in response to the set of adjustment values, thus generating a set of normalized detector signals.
Certain embodiments of the present invention afford an approach to generating and displaying a plurality of X-ray image frames at an increased frame rate by reducing the signal conversion time for a solid-state detector of an X-ray system.